This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Electrical activities of living cells are coupled to the fast long-distance information flow in the nervous system and are subject of great scientific and medical interests. Nevertheless, the existing observation methods for electrical activity have various limitations. We propose that membrane electromotility (MEM), which arise from the intrinsic property of cell membrane, can be used as an optical indicator of cell electrical activity. Since it relies on an endogenous mechanism, this approach is free of bleaching and toxicity. Also the sample preparation will be simpler. But optical detection of MEM signals poses a significant challenge due to its extremely small size (~nm). To meet this challange, we propose to build 1) a wide-field transmission phase microscope using a low-coherence source and (2) a reflection phase microscope with depth-resolved phase measurement capability at multiple lateral locations simultaneously, with the aim to effectively measure and understand the MEM signal in mammalian cells.